This invention relates generally to presses for compacting powder into shapes, and particularly but not exclusively to such presses for the compaction of ceramic nuclear fuel powder into pellets which are to be stacked in sealed containers forming part of nuclear fuel assemblies for nuclear reactors.
Although commercially available powder presses can be adapted for use with ceramic nuclear fuel powder, it has proved difficult to provide against loss of powder due to the fact that the compacts are removed from the press dies in a direction against the inward flow of powder, necessitating complicated die filling devices such as shoes or sweep arms which are difficult to seal against loss of feed powder. Most commercial presses are used for confectionery or with other common and inexpensive powders so that some loss of powder is acceptable, but where the powder is a fissile ceramic material, particularly one containing plutonium, any loss of powder is to be avoided, on cost, safety and security grounds.
Commercial presses fall into two general kinds, the mechanical press, with its set stroke, and the hydraulic. The mechanical kind provides compacts of constant depth but green density is variable unless constant fill is effective, and this requires very expensive metering mechanism. The hydraulic press enables a close control over green density, but this is at the expense of constant depth unless the said expensive constant fill is provided. Because constant green density is important so that subsequent diameter grinding after sintering is reduced or eliminated, hydraulic pressing is necessary to compensate for die fill variation. Other variants affecting green density are intergranular and die-wall friction, and entrapped air. Intergranular friction has a small effect on green density variation compared with die wall friction but often granules are lubricated to provide the die wall lubrication required. Die wall lubrication separately is a much more economical operation and does not introduce contaminants into the granules. Provided the depth/diameter ratio is not too large and if ample time is allowed for a compaction cycle, variations of green density due to entrapped air can also be reduced to negligible proportions. By reducing or eliminating green density variation, diameter grinding, which is expensive and time consuming, can be reduced or eliminated.